Semiconductor laser

ABSTRACT

The invention has such a double hetero structure ( 11 ) that an active layer ( 3 ) is sandwiched by an n-type clad layer ( 2 ) and a p-type clad layer ( 4 ) on a semiconductor substrate ( 1 ) made of GaAs. In the p-type clad layer ( 4 ), for example, an n-type current constriction layer ( 6 ) consisting of at least two layers is provided in such a configuration that a first layer ( 6   a ) thereof closer to the active layer is made of a material having almost the same refractive index as the p-type clad layer and a second layer ( 6   b ) thereof farther from the active layer is made of a material having a smaller refractive index than the first layer ( 6   a ). By this configuration, a self-excitement type and high-power semiconductor laser can be obtained which operates in a stable manner up to a high power without generating a kink while being self-excited at a low power. Another embodiment of the invention comprises a current constriction layer having an n-type in which a stripe trench is formed in the p-type clad layer, and a light confinement layer having a smaller refractive index than the p-type clad layer is formed at the current constriction layer facing the active layer, so as to be of a p-type or non-doped type. By this configuration, a semiconductor laser can be obtained which operates up to a high power without generating a kink.

FIELD OF THE INVENTION

The present invention relates to a semiconductor laser used as a lightsource for an optical disk device such as a CD-ROM, a CD-R, or aDVD-ROM, a high-definition laser beam printer (LBP), a laser pointer, orthe like. More specifically, it relates to such a semiconductor laserthat can oscillate at a high power without generating a kink byconstricting a current by use of the current constriction layer as wellas confining the light as much as possible, or that can oscillate at ahigh power without generating a kink while being self-excited at a lowpower by confining the light as well to some extent.

BACKGROUND OF THE INVENTION

A self-alignment type semiconductor laser provided with a lightconfinement effect due to a current constriction layer has such aconstruction as shown in FIG. 4 as an example. That is, as shown in FIG.4, on a substrate 21 made of, e.g., an n-type GaAs are sequentiallygrown epitaxially an n-type clad layer 22 made of, e.g., n-typeAl_(0.6)Ga_(0.4)As, an active layer 23 made of non-dopedAl_(0.2)Ga_(0.8)As, a p-type first clad layer 24 a made of p-typeAl_(0.6)Ga_(0.4)As, an etching stopping layer 25, a current constrictionlayer 26 made of, e.g., n-type Al_(0.7)Ga_(0.3)As, a p-type second cladlayer 24 b made of p-type Al_(0.6)Ga_(0.4)As, and a p-type contact layer27 made of GaAs, on the top surface of which is formed a p sideelectrode 28 and on the back surface of the GaAs substrate 21 is formedan n side electrode 29, so that the resultant wafer is subdivided intochips by cleavage or the like to thereby form a semiconductor laser (LD)chip having a construction shown in FIG. 4.

This construction employs, to use the laser for write-in operations orthe like, a method for disposing the current constriction layer near theactive layer or enlarge the mixed-crystal ratio of Al in the currentconstriction layer to provide an effective difference in refractiveindex in order to enhance the light confinement effect, thus oscillatinga laser at a high power.

Also, to use the laser for read-out operations or the like, such amethod is required to be employed that the noise is reduced at a lowpower and, for self excitement, the mixed-crystal ratio of Al in thecurrent constriction layer is reduced or the current constriction layeris disposed distant from the active layer to thereby relax the lightconfinement effect in order to spread the light, thus enabling forming asupersaturating absorption layer outside a current implanting region inthe active layer.

As mentioned above, in order to enhance the light confinement effectsand oscillate a laser at a high power, the current constriction layermust be disposed near the active layer as much as possible or themixed-crystal ratio of Al in the current constriction layer made of anAlGaAs-based compound semiconductor must be enlarged to thereby reducethe refractive index. If the mixed-crystal ratio of Al is enlarged,however, the exposed surface of the current constriction layer after astripe trench is formed therein easily corrodes because Al is veryeasily oxidized, thus suffering from a problem that a clean mono-crystalsemiconductor layer cannot easily be grown when a semiconductor layer isgrown again. Although by, in particular, forming beforehand a protectivelayer such as made of GaAs on the top-most surface of the currentconstriction layer, thermal etching can be carried out before there-growing, thus providing a clean layer, the side walls of the stripetrench cannot be cleaned in such a way, so that the semiconductor layeris liable to be poly-crystallized to thereby flow a leakage current andso increase the threshold current value, thus leading to a problem of anincreased electric resistance due to poly-crystallization and also arise in the operation current.

Since the current constriction layer, on the other hand, has aconductivity type different from that of its surrounding clad layer tothereby prevent a current flow by the reverse-biased pn junction, thepn-junction portion has a depletion layer formed thereon due to thereverse biasing, so that as shown in FIG. 5, if the current constrictionlayer 26 is formed too close to the active layer 23, the depletion layer(refer to C in FIG. 5) reaches the active layer 23. If the depletionlayer C reaches to the active layer 23, as shown in FIG. 5, a current Iflows to the portion of the current constriction layer 26 where nostripe trench is formed to disable from constricting the current, thusleading to a problem that the invalid current flows through the activelayer.

Although to use a laser for both write-in and read-out operations, onthe other hand, the laser must oscillate at a high power withoutgenerating a kink while being self-excited at a low power to therebyreduce the noise, as mentioned above, there is a trade-off relationshipbetween self-excitement at a low power and obtaining a high power and,therefore, both requirements cannot be satisfied at the same time, sothat a self-excitement type semiconductor laser, which has a largefluctuation in power, suffers from a phenomenon called a kink that thepower drops during the process of increasing the operation current,leading to a problem of difficulty in obtaining of a high power. Asemiconductor laser for obtaining a high power, on the other hand,cannot be self-excited, leading to a problem that the noise cannot besuppressed.

In view of the above, it is an object of the invention to provide aself-excitement type, high-power semiconductor laser that can operate ina stable manner even at a high power without generating a kink whilebeing self-excited at a low power.

It is another object of the invention to provide a high-powersemiconductor laser which can operates in a stable manner even at a highpower with no kink generated without enlarging so much the mixed-crystalratio of Al in the current constriction layer and also with preventing adepletion layer due to a pn junction of the current constriction layerfrom reaching the active layer.

SUMMARY OF THE INVENTION

A semiconductor laser according to the invention comprises an activelayer sandwiched by n-type and p-type clad layers in such a constructionthat either one of the above-mentioned clad layers is provided with acurrent constriction layer consisting of at least two layers for currentconfinement and light confinement, the first layer of the currentconstriction layer closer to the above-mentioned active layer has aconductivity type different from that of the clad layer provided withthe current constriction layer and being made of a material havingalmost the same refractive index as that of the clad layer and thesecond layer of the above-mentioned current constriction layer fartherfrom the active layer being made of a material having a refractive indexsmaller than that of the first layer.

By this construction, the first layer closer to the active layer hasalmost the same refractive index as that of the clad layer to therebyeliminate the light confinement effect, thus serving as a layer only forconstricting the current. The second layer farther from the activelayer, on the other hand, has a smaller refractive index than the firstlayer and has the light confinement effect. In this case, the light isemitted from the current implanting region constricted by the firstlayer closer to the active layer and then the second layer confines thelight in the light emitting region from which the light was oscillated,so that the light emitting region in which the light is confined becomeslarger than the region to which a carrier is implanted to emit the lightthereby permitting thus enlarged portion of the active layer to act asan supersaturating absorption layer. Accordingly, the light can beconfined in an enhanced manner while reserving that supersaturatingabsorption layer, to sufficiently confine the light duringself-excitement by use of the supersaturating absorption layer, thusobtaining a high power without generating a kink. Note here that thesecond layer does not have to be different in conductivity type from thesurrounding clad layer but may have the same conductivity type if thefirst layer can confine the current sufficiently.

In the event that, for example, AlGaAs based compound semiconductor foremitting the infrared light or InGaAlP based compound semiconductor foremitting the red light forms a double hetero construction in which theactive layer is sandwiched by n-type and p-type clad layers having alarger band gap than the active layer, the first layer of the currentconstriction layer is made of a material having almost the samecomposition as that of the clad layer of the AlGaAs based or InGaALPbased compound semiconductor, and the second layer is made of a materialhaving an enlarged mixed-crystal ratio of Al to thereby provide anenlarged difference in refractive index, so that the larger themixed-crystal ratio of Al or the closer to the active layer, the morethe layer has the light confinement effect.

It is preferably that the first and second layers are so formed as tofunction mainly as a current confinement layer and a light confinementlayer respectively and also the stripe trench provided to the firstlayer is so formed as to be smaller in width than that provided to thesecond layer, because the supersaturating absorption layer can bereserved securely. That is, if the stripe trench is formed notperpendicular to the surface of the semiconductor layer but is formed tohave an inclined surface with respect to that, the stripe width of thefirst layer closer to the active layer is made smaller than that of thesecond layer. In this case, however, the two stripe trenches may beetched in different patterns to form the first layer narrower than thesecond layer.

Even if the stripe trenches are formed so as to have an inclined surfacewith respect to the width direction of the current constriction layerand the inclined surface of the first layer is formed so as to have asmaller inclination angle than that of the second layer, the stripetrench of the first layer has an even smaller width than the secondlayer, so that the supersaturating absorption layer can be expanded.Here, the inclination angle of the inclined surface refers to an angle θof the side wall of the stripe trench with respect to the growingsurface of the semiconductor layer (see FIG. 1(b)).

The semiconductor laser according to the invention may have anotherconstruction that has a double hetero structure in which the activelayer is sandwiched by n-type and p-type clad layers and either one ofthese clad layers is provided with a current constriction layer with astripe trench having a different conductivity type from that of thisclad layer and also at the current constriction layer on the side of theactive layer a light confinement layer having s smaller refractive indexthan this clad layer is formed without doping or is formed so as to havethe same conductivity type as this clad layer.

By this construction, the light confinement layer having a smallerrefractive index is formed without doping or so as to have the sameconductivity type as the clad layer, so that the only the lightconfinement effect can be obtained without forming a reverse-biased pnjunction. Accordingly, the light confinement layer can be disposed closeto the active layer unlimitedly. On the side of that light confinementlayer opposite to the active layer, on the other hand, a currentconstriction layer having a different conductivity type from that of theclad layer is provided to thereby inhibit a current by thereverse-biased pn junction. A depletion region due to this reversebiasing would spread to the light confinement layer but can becontrolled not to reach the active layer by adjusting the thickness ofthe light confinement layer, thus preventing a leakage current fromflowing therethrough. As a result, it is possible to enhance the lightconfinement effect while sufficiently preventing the leakage currentfrom flowing, thus providing stable operations up to a high power.

If the current constriction layer is formed of a semiconductor layerhaving the same refractive index as the light confinement layer, it ispossible for this light confinement layer, even if it is thin, toprevent a depletion layer due to the reverse biasing from reaching theactive layer while sufficiently confining the light together with thecurrent constriction layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) to 1(b) are cross-sectional views explaining one embodimentof a semiconductor laser according to the present invention.

FIG. 2 is a cross-sectional view explaining a variant of thesemiconductor laser of FIG. 1.

FIG. 3 is a cross-sectional view explaining another embodiment of thesemiconductor laser according to the present invention.

FIG. 4 is a cross-sectional view explaining a conventional self-excitingtype semiconductor laser.

FIG. 5 is a cross-sectional view explaining a problem with a case wherea current constriction layer is disposed too close to an active layer ina construction of FIG. 4.

DETAILED DESCRIPTION

The following will describe embodiments of a semiconductor laseraccording to the present invention with reference to the drawings. Asshown in a cross-sectional view of one embodiment thereof in FIGS. 1(a)and 1(b), the semiconductor laser according to the present invention hassuch a double hetero structure 11 on a semiconductor substrate 1 madeof, e.g., n-type GaAs that an active layer 3 is sandwiched by n-type andp-type clad layers 2 and 4. Further, for example, the p-type clad layer4 is provided with a current constriction layer 6 consisting of at leasttwo layers having a conductivity type (e.g., n-type) different from theconductivity type (e.g., p-type) of this clad layer 4, in such aconfiguration that a first layer 6 a of the current constriction layer 6which layer is closer the active layer 3 is made of a material havingalmost the same refractive index as that of the p-type clad layer 4 anda second layer 6 b of the current constriction layer 6 which layer isfarther from the active layer 3 is made of a material having a smallerrefractive index than the first layer 6 a.

In such a case as shown in FIG. 1(a), the double hetero structure 11 hassuch a stacked-layer configuration that comprises the n-type clad layer2 made of, e.g. n-type Al_(x)Ga_(1-x)As (0.3≦x≦0.8, e.g. x=0.6), theactive layer 3 made of non-doped, n-type, or p-type Al_(y)Ga_(1-y)As(0≦y≦0.3, e.g. y=0.15), and a p-type first clad layer 4 a made of p-typeAl_(x)Ga_(1-x)As. Specifically, in order to determine the material ofthe active layer 3 based on a band gap corresponding to a desired lightemission wavelength to confine a carrier in the active layer 3, thisdouble hetero structure 11 is sandwiched by the clad layers 2 and 4which have a larger band gap than that material. Therefore, depending onthe desired wavelength, in place of an AlGaAs based compound, an InGaAlPbased compound or any other semiconductor may be used. Note here thatbesides being the bulk layer, the active layer 3 may be of amultiple-quantum construction of AlGaAs-AlGaAs (AlGaAs-GaAs) thatalternately stacks a well layer made of Al_(0.1)Ga_(0.9)As or GaAs and abarrier layer made of Al_(0.3)Ga_(0.7)As.

The p-type clad layer 4 is divided into a first clad layer 4 a and asecond clad layer 4 b with the current constriction layer 6 formedtherebetween via the etching stop layer 5. The etching stop layer 5 isprovided for stopping further etching when a stripe trench 6 c is formedin the current constriction layer 6 and only needs to be formed by sucha composition as to enable local etching of the current constrictionlayer 6, e.g. a non-doped or p-type composition of Al_(a)Ga_(1-a)As(0≦a≦1, a≠z, r) or InGa_(1-b)Al_(b)P (0≦b≦0.5).

The current constriction layer 6 is divided into a first layer 6 a madeof, e.g. n-type Al_(z)Ga_(1-z)As (0.3≦z≦0.8, z is nearly equal to x) anda second layer 6 b made of Al_(r)ga_(1-r)As (0.4≦r≦1, x<r, z<r), withthe stripe trench 6 c formed through both layers (which trench appearsto be extending perpendicular to the paper surface because the figureshows a cross-sectional view in a direction perpendicular to thestripe).

This first layer 6 a is made of a material having almost the samecomposition as the p-type clad layer 4 and so has little lightconfinement effect. However, as can be seen from the fact that it isformed as an n-type layer in the p-type clad layer 4, this first layer 6a has a different conductivity type from the clad layer 4 provided withthe current constriction layer 6 and so serves to inhibit a currentflow, thus permitting a current to flow only through the portion wherethe stripe trench 6 c is formed. That is, it has an action to narrow acurrent only to the portion of the stripe trench 6 c. The second layer 6b, on the other hand, has a larger mixed-crystal ratio of Al of theAlGaAs based compound than the first layer 6 a and is made of a materialhaving a smaller refractive index so as to have a light confinementeffect and, at the same time, is formed to be an n-type compound so asto have a current constriction effect in corporation with the firstlayer 6 a. In an example shown in FIG. 1(a), although the second layer 6b is also formed as an n-type layer, the first layer 6 a alone canconstrict the current if it has a width of about 0.2 μm, in which casethe conductivity type of the second layer 6 b is not limited.

The current constriction layer 6 is thus comprised of at least twolayers in such a configuration that the first layer 6 a closer to theactive layer 3 only has a current confining effect without confining thelight, to permit the second layer 6 b to have the light confinementeffect. Besides, although the above-mentioned stripe trench 6 c can beetched so as to have almost perpendicular side walls of its own byadjusting the etching conditions, the inclination angle θ of the sidewalls is decreased if the ratio of a hydrogen peroxide component isdecreased in, for example, a sulfuric acid-based etching solution, sothat as shown in FIG. 1(b), the side walls are formed so as to have aninclined surface with respect to the surface of the stackedsemiconductor layers. Accordingly, the width of the stripe trench 6 c isformed so as to have a width B in the second layer 6 b larger than awidth A in the first layer 6 a.

On the current constriction layer 6 and also on the etching stop layer 5exposed through the stripe trench 6 c is grown a p-type second cladlayer 4 b having the same composition as the p-type first clad layer 4a, on which is further provided a contact layer 7 made of p-type GaAs,on the surface of which is provided a p-type electrode 8 and on the backsurface of semiconductor substrate 1 is provided an n-type electrode 9,so that the structure can be subdivided into chips by cleavage and thelike, thus obtaining a semiconductor laser having the construction shownin FIG. 1. Although in the above-mentioned example the n-type GaAssubstrate has been employed, a p-type substrate may also be used toobtain the same results by reversing the conductivity types of all thosesemiconductor layers and also the current constriction layer may beformed below the active layer.

To manufacture the semiconductor laser shown in FIG. 1(a), first on thesemiconductor substrate 1 made of n-type GaAs, by using an MOCVD or MBEmethod, the n-type clad layer 2 made of n-type Al_(0.6)Ga_(0.4)As with athickness of about 1 μm, the active layer 3 made of non-dopedAl_(0.15)Ga_(0.85)As with a thickness of about 0.1 μm, the first cladlayer 4 a made of p-type Al_(0.6)Ga_(0.4)As with a thickness of about0.15 μm, the etching stop layer 5 made of non-dopedIn_(0.5)Ga_(0.4)Al_(0.1)P with a thickness of about a few tens ofnano-meters, the first layer 6 a of the current constriction layer 6made of n-type Al_(0.6)Ga_(0.4)As with a thickness of about 0.3 μm, thesecond layer 6 b of the current constriction layer 6 made of n-typeAl_(0.7)Ga_(0.3)As with a thickness of about 0.3 μm, and an oxidationpreventing layer made of n-type GaAs (not shown) with a thickness ofabout 0.03 μm are stacked sequentially.

Next, this device in process is masked with a photo-resist except aformation-reserved portion of the stripe trench 6 c and etched with, forexample, a sulfuric acid-based solution to form the stripe trench 6 c,thus forming the stripe trench 6 c. This solution cannot etchIn_(0.5)Ga_(0.4)Al_(0.1)P, so that etching is stopped at the etchingstop layer 5 to prevent the p-type first clad layer 4 a from beingetched, thus etching only the current constriction layer 6 to apredetermined width. In this case, by decreasing the hydrogen peroxidecomponent of the sulfuric acid-based solution, the side walls of thestripe trench 6 c has a smaller inclination angle θ, thus obtaining amoderately inclined surface.

Then, the device in process is put in a reactive vessel such as a MOCVDapparatus again to conduct thermal cleaning to thereby evaporate theoxidation preventing layer made of GaAs (not shown), thus stacking boththe p-type second clad layer 4 b made of p-type Al_(0.6)Ga_(0.4)As andthe contact layer 7 made of p-type GaAs to about 1 μm. Then, therespective p-type and n-type electrodes 8 and 9 are formed by, forexample, evaporation and then subdivided into chips, thus obtaining asemiconductor laser chip shown in FIG. 1(a).

According to the present invention, the current constriction layer 6 is,as mentioned above, comprised of at least two layers in such aconfiguration that the first layer 6 a thereof closer to the activelayer 3 has only the current confinement effect without confining thelight and the second layer 6 b thereof acts to confine the light.Accordingly, as shown in an expanded portion of the current constrictionlayer of FIG. 1(b), a portion A where a carrier is confined in theactive layer 3 to thereby emit the light does not agree with a portion Bwhere the light is confined by the second layer 6 b of the currentconstriction layer 6 (which light is actually confined by the secondlayer 6 b as distant from the active layer 3, so that the portion Bspreads laterally a little), so that the light confinement region Bbecomes wider than the other. Accordingly, even if the light is confinedin an enhanced manner, a non-overlapping portion 3 b of the lightconfinement region B and the light emitting region A due to the carrierof the active layer 3 is supersaturated, to provide a repetitive regionacting as a light emitting and light non-emitting regions alternatelyduring a low-power operation, thus being self-excited. During ahigh-power operation, on the other hand, it always acts as a lightemitting region, to oscillate a light in a stable manner throughout theoperation region, thus obtaining a high power without a kink.

Although in the above-mentioned example, the side walls of the stripetrench 6 c of the current constriction layer 6 has been formed to havealmost the same inclination angle through the two layers, as shown inFIG. 2, the etching conditions may be changed to form it with a moderateinclination angle of the first layer 6 a and a steep inclination angleof the second layer 6 b in order to enlarge the non-overlapping portionof the above-mentioned light emitting region A and light confinementregion B, thus increasing the width of the supersaturating absorptionlayer. As a result, the device can be self-excited easier at a lowpower, thus enhancing the light confinement effect. Also, although notshown, the first layer 6 a and the second layer 6 b may be patterned inetching thereof to form the stripe trench 6 c, to thereby change thewidth or, similarly, by making self-excitement easier to occur, thelight confinement effect can be enhanced. The other elements in FIG. 2are not explained here because they are indicated by the same referencenumbers as those in FIG. 1(a).

In the self-excitement type semiconductor laser according to theinvention using a current constriction layer to have a light confinementeffect, current constriction and light confinement operations can beperformed separately from each other, so that even if the light isconfined in an enhanced manner, an supersaturating absorption layer canbe reserved, to thereby satisfy both requirements of self-excitement anda high power operation without a kink. As a result, such a semiconductorlaser can be obtained that has low-noise characteristics free the noisedue to the return light and also has high-power characteristics.

A high-power semiconductor laser according to the present inventioncomprises, as shown in a cross-sectional view of its embodiment of FIG.3, has such a double hetero structure 11 that on the semiconductorsubstrate 1 made of, e.g., n-type GaAs the active layer 3 is sandwichedby n-type and p-type clad layers 2 and 4(4 a). In, for example, thep-type clad layer 4 (4 a and 4 b) is provided the current constrictionlayer 6 in which the stripe trench 6 c is formed which has a differentconductivity type (e.g., n-type) from the conductivity type (e.g.,p-type) of that clad layer 4, on the active layer 3 side of that currentconstriction layer 6 is formed a light confinement layer 10 which has asmaller refractive index than the p-type clad layer and also which hasthe same conductivity type as that p-type clad layer 4 or is non-doped.

The double hetero structure 11 is the same as that of theabove-mentioned case shown in FIG. 1(a) except that the currentconstriction layer does not consists of two layers but comprises thelight confinement layer 10 and the current constriction layer 6.

In the case shown in FIG. 3, both the light confinement layer 10 and thecurrent constriction layer 6 are made of Al_(s)Ga_(1-s)As (0.4≦s≦0.8,x<s) and have the stripe trench 6 c formed therein (which appears toextend perpendicular to the paper surface because the figure shows thecross-sectional view in a direction perpendicular to the stripe). Thatis, it is formed to have a larger mixed-crystal ratio s of Al than theclad layer 4 to thereby have a reduced refractive index, thus acting toconfine the light to the side of the active layer 3.

On the other hand, the light confinement layer 10 is formed to have thesame conductivity type as the surrounding clad layer, i.e. p-type ornon-doped type in the case shown in FIG. 3, while the currentconstriction layer 6 is formed to have the opposite conductivity type,i.e. n-type. Accordingly, at the boundary between this currentconstriction layer 6 and the light confinement layer 10 is formed areverse-biased pn junction surface to inhibit a current flow, so that acurrent flows only through a portion where the stripe trench 6 c isformed, thus narrowing the current. If a reverse bias is applied to thispn junction surface, as indicated by a broken line in FIG. 3, thedepletion layer C is formed on both sides of the pn junction. The lightconfinement layer 10 is so formed that this depletion layer C has such athickness as not to reach the active layer 3. If the light confinementlayer 10 is too thick, the current injecting region is liable to spreadfrom the stripe trench, which is not preferable. For example, the lightconfinement layer 10 is formed to a thickness of about 0.05-0.3 μm andthe current constriction layer 6, to a thickness of about 0.2-0.5 μm.

Explanation of the other structures and the manufacturing procedure isomitted here because they are the same as the above-mentioned exampleshown in FIG. 1(a) and indicated by the same reference numerals.

According to this embodiment, besides the current constriction layer 6,on the side of the active layer, the light confinement layer 10 having alight-confinement action is formed which has a non-doped type or thesame conductivity type as the clad layer 4 surrounding that currentconstriction layer 6 and also which has a smaller refractive index thanthe clad layer. Accordingly, even if the light confinement layer 10comes close to the active layer 3, the depletion layer does not breakthrough into the active layer, thus enabling enhancing the lightconfinement effect due to the proximity to the active layer 3. As aresult, without reducing the refractive index by unnecessarily enlargingthe mixed-crystal ratio of Al of the current constriction layer 6, thelight can be confined sufficiently to improve the crystallinity, thusimproving the light emission characteristics including a reduced currentvalue and also enhancing the light confinement effect for stableoperations up to a high power.

By this embodiment employing an AlGaAs based compound or InGaAlP basedcompound semiconductor, in the self-alignment type semiconductor laserwhich provides a high power by confining the light, since the lightconfinement layer besides the current constriction layer is formed tohave a non-doped type or the same conductivity type as the surroundingclad layer, the light confinement layer can be disposed close to theactive layer arbitrarily to enhance the light confinement effect withoutenhancing it by unnecessarily enlarging the mixed-crystal ratio of Al.As a result, a stable, high-power semiconductor laser can be obtainedwhich does not generate a kink.

Although preferred examples have been described in some detail it is tobe understood that certain changes can be made by those skilled in theart without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A semiconductor laser comprising: an n-type cladlayer; a p-type clad layer; an active layer sandwiched by said n-typeclad layer and said p-type clad layer; and a current constriction layerfor current confinement and light confinement consisting of at least twolayers which is disposed in either of said n-type clad layer and saidp-type clad layer, wherein a first layer of said current constrictionlayer closer to said active layer has a different conductivity type froma conductivity type of either of said clad layers in which said currentconstriction layer is provided and is made of a material having almostthe same refractive index as said clad layer, the refractive index ofsaid first layer being smaller than that of said active layer, andwherein a second layer of said current constriction layer farther fromsaid active layer is made of a material having a smaller refractiveindex than said first layer.
 2. The semiconductor laser of claim 1,wherein said first layer of said current constriction layer is formed tofunction mainly as a current confinement layer and said second layerthereof is formed to function mainly as a light confinement layer and awidth of a stripe trench for injecting current provided in said firstlayer is smaller than a width of a stripe trench provided in said secondlayer.
 3. The semiconductor laser of claim 2, wherein said stripe trenchis formed so as to have an inclined surface with respect to awidth-direction of said current constriction layer, so that a width ofsaid stripe trench for injecting current provided in said first layer issmaller than a width of said stripe trench provided in said secondlayer.
 4. The semiconductor laser of claim 3, wherein said inclinedsurface of said first layer has a smaller inclination angle than saidsecond layer.
 5. The semiconductor laser of claim 2, wherein said stripetrench in said first layer and said stripe trench in said second layerare provided in different steps, so that the width of said stripe trenchprovided in said first layer is smaller than the width of said stripetrench provided in said second layer.